So simply my question is if you could fit a s4 turbo core/compressor onto a s5 turbine housing.
I was wanting to upgrade to a s5 turbo because of the better waste gate design, plus my s4 turbine housing is cracked.

 

i believe it would drop right in

 

There is one minor dimension, that would make a huge issue, that requires you to measure the S4 core. If I recall correctly, there are/were some early S4 turbos that had 2 heat shields or a thicker heat shield to protect the core from the turbine. Ill look to see if I can find a link to this info. Then just use the S5 wastegate actuator.